1 Answer
1

In my experience, for Hochdeutsch, "-lein" nowadays is often the exception that you would fall back to in case the more common "-chen" doesn't work, cf. "Bächlein" (because "Bächchen" would be impossible to pronounce even for native speakers).

An a/o/u vowel often changes to their corresponding umlaut:

Bach -> Bächlein

Brot -> Brötchen

Punkt -> Pünktchen

Additionally, if the last letter is a vowel, it is often omitted when forming a diminutive:

Schraube -> Schräubchen

Katze -> Kätzchen

There's also a separate diminutive "-i/-y" form mainly used with person names:

Hans -> Hansi

Fred -> Fredi

Thomas -> Tommy

Some examples of both Hochdeutsch and dialects can be found on Wikipedia